Updated: 6/17/2021

Oncogenic Osteomalacia

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  • summary
    • Oncogenic Osteomalacia is a paraneoplastic syndrome of renal phosphate wasting caused by a bone or soft tissue tumor. Patients may present with osteopenia and pathologic fractures.
    • Diagnosis is made with PET/SPECT scans to identify the location of the tumor. 
    • Treatment involves identifying and resecting the tumor. 
  • Epidemiology
    • Demographics
      • age bracket
        • age of onset is late childhood to early adulthood
  • Etiology
    • Pathophysiology
      • the tumor secretes a humoral factor ("phosphatonin") that affects the proximal renal tubules
      • reduces calcitriol production in the kidney and inhibits phosphate transport
      • leads to increased renal phosphate excretion, hypophosphatemia and osteomalacia
      • types of tumors that cause oncologic osteomalacia (known as phosphaturic mesenchymal tumor)
        • benign tumors (more common)
          • phosphaturic mesenchymal tumors (mixed connective tissue variant) e.g. hemangiopericytoma (commonest cause)
          • osteoblastoma-like tumors
          • ossifying fibrous tumors
          • nonossifying fibrous tumors
        • malignant causes (rare)
          • osteosarcoma
          • fibrosarcoma
    • Genetics
      • mutations
        • phosphatonin gene is FGF23
    • Associated conditions
      • orthopaedic manifestations
        • pathological fractures of long bones and vertebrae
  • Presentation
    • Symptoms
      • generalized bone and muscle pain
      • fractures of long bones, ribs and vertebrae
      • proximal muscle weakness
      • fatigue
  • Imaging
    • Radiographs
      • findings
        • diffuse osteopenia
        • Looser's zones (pseudofractures)
    • Octrotide scans (radiolabeled somatostatin analog)
      • gallium-68 DOTA-octreotate PET scan
      • indium-111 pentetreotide SPECT/CT
      • indications
        • to identify primary tumors when TIO is suspected
        • will only identify tumors expressing somatostatin receptors
  • Studies
    • Lab Values
      Serum Ca
      Serum P
      Alk phos
      PTH
      25-(OH)vit D
      1,25-(OH)vit D
      Urinary Ca
      Osteomalacia
      Low 
      Low
      High
      High
      Low 
      Low 
      Low
      Osteoporosis
      Normal
      Normal
      Variable
      Normal
      Normal
      Normal
      Normal
      Tumor induced osteomalacia
      Low 
      Very low
      Low
      Low
      Low
      Low
      Low
      Osteopetrosis

      Normal
      High
      Normal
      Normal
      Normal
      Normal
      Normal
  • Treatment
    • Nonoperative
      • phosphate supplementation with 1,25-dihydroxyvitamin D
    • Operative
      • tumor removal
        • outcomes
          • resolution of hypophosphatemia and low vitamin D levels within hours of resection
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